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1286 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 4, AUGUST 2008

Soft X-Ray Laser Interferometry ofa Dense Plasma Jet

Jonathan Grava, Michael A. Purvis, Jorge Filevich, Mario C. Marconi, James Dunn,Stephen J. Moon, V. N. Shlyaptsev, and Jorge J. Rocca, Fellow, IEEE

AbstractSoft X-ray laser interferograms were acquired tomap the evolution of a dense plasma jet created by the laser irra-diation of a solid copper triangular target. The plasma is observedto rapidly expand along the symmetry plane of the target, forminga narrow plasma plume with measured electron densities of up to1.2 10

20 cm3.

Index TermsPlasma diagnostics, plasma jets, soft X-ray laser,soft X-ray laser interferometry.

U NDERSTANDING the formation and evolution of jetsis of significant interest both in astrophysical [1] andlaboratory plasmas where jets can result from the irradiation of

solid targets with intense lasers [1][3]. Detailed experimental

data is needed to aid the understanding of the physics of

plasma jets and to validate the codes used to model them.

Laboratory plasma jets can have large density gradients that

cause optical laser probes to be refracted. In contrast, short

wavelength light can propagate in dense plasmas with greatly

reduced refraction, probing regions that cannot be accessed

with optical lasers. In particular, soft X-ray laser interfer-

ometry can yield detailed 2-D maps of the electron density.

Our group has previously used soft X-ray interferometry at

46.9 [4], [5] and 14.7 nm [6] to study a variety of dense

plasmas.

Herein, we present a snapshot of the density distribution in

a dense plasma jet. The jet was created by irradiating 90

triangular Cu grooves at an intensity of 1.1 1012 W cm2

with = 800-nm Ti:Sapphire laser pulses that were 120 ps

in duration. The grooves were 220 m deep, 500 m wide,

and 1 mm long. This laser-created plasma was probed along

the length of the groove with a 46.9-nm beam generated by a

capillary discharge-pumped tabletop Ne-like Ar amplifier [6],

Manuscript received December 1, 2007. This work was supported by theNational Nuclear Security Administration under the Stewardship Science Aca-demic Alliances Program through the U.S. Department of Energy ResearchGrant DE-FG52-06NA26152.

J. Grava, M. A. Purvis, J. Filevich, M. C. Marconi, and J. J. Rocca arewith the Department of Electrical and Computer Engineering, Colorado StateUniversity, Fort Collins, CO 80523 USA (e-mail: rocca@engr.colostate.edu).

J. Dunn and S. J. Moon are with Lawrence Livermore National Laboratory,Livermore, CA 94551 USA.

V. N. Shlyaptsev is with the Department of Applied Science, University ofCalifornia-Davis, Livermore, CA 94551 USA.

Digital Object Identifier 10.1109/TPS.2008.924402

which delivers laser pulses of 1-ns duration. The capillary

discharge laser was combined with a soft X-ray amplitude

division interferometer that uses diffraction gratings to split

and recombine the beam [7]. Varying the delay between the

46.9-nm probe pulse and the laser irradiation pulse enabled the

acquisition of high-contrast interferograms at different times

during the plasma evolution.

The interferograms show that a very narrow plasma jet with a

large length to width ratio (> 10 : 1) is formed at the bottom of

the groove. The jet expands nearly 300 m along the symmetryplane of the cavity within the first nanosecond after laser irradi-

ation. Fig. 1(a) shows a soft X-ray laser interferogram obtained

4.2 ns after the irradiation of the target. The shift in the fringes

indicates that, at this time, the plasma jet has already expanded

outside the groove, reaching a length of600 m, while simul-

taneously broadening to a width of65 m. The analysis of the

interferogram [Fig. 1(b)] shows that, at a distance of100 m

from the bottom of the cavity measured along the symmetry

plane, the electron density exceeds 1.2 1020 cm3.

Two-dimensional simulations performed by using the hy-

drodynamic code HYDRA show that the jet formation is ini-

tiated by accelerated material ablated from the vertex that is

augmented by the continual sequential arrival of wall materialalong the symmetry plane, where it collides and is redirected

outward. Early on, radiation is found to play a significant

role in cooling the plasma, which reduces the initial pressure,

maintaining a collimated jet over an extended period of time.

Later in the evolution, the pressure in the jet is no longer

counterbalanced by the momentum of plasma ablated from the

sidewalls and causes it to widen. The collision of the expanding

plasma jet with the plasma generated at the walls forms plasma

side lobes.

The results illustrate that soft X-ray laser interferometry is

a powerful tool to generate 2-D maps of the electron density

in dense large-scale plasmas that cannot be probed with optical

lasers.

ACKNOWLEDGMENT

The authors would like to thank the NSF ERC Center for

Extreme Ultraviolet Science and Technology for allowing the

use of their facilities under Award EEC-0310717. Part of this

work was performed under the auspices of the U.S. Department

of Energy by Lawrence Livermore National Laboratory under

Contract DE-AC52-07NA27344.

0093-3813/$25.00 2008 IEEE

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8/3/2019 Jonathan Grava et al- Soft X-Ray Laser Interferometry of a Dense Plasma Jet

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GRAVA et al.: SOFT X-RAY LASER INTERFEROMETRY OF DENSE PLASMA JET 1287

Fig. 1. (a) Soft X-ray interferogram and (b) corresponding plasma density map of a plasma jet created by the laser irradiation of a triangular copper groove.The interferogram is taken 4.2 ns after the irradiation of the target surface (outlined on the figure by the solid black line) by a 120-ps-duration optical laser pulsewith an intensity of1.1 1012 W cm2.

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